SoftRAID Mirror volume won't mount

This is a known issue with Time Machine.

Try these steps:

Step 1. Disconnect Time Machine from the volume
If the volume mounts at all (even briefly):
System Settings → General → Time Machine
Remove this disk as a Time Machine destination
This releases Time Machine’s exclusive claim on the volume.
If it won’t auto mount:
Do this after the next step once it does.

Step 2. Boot once into Safe Mode
This is very important on Apple Silicon.
Safe Mode:
disables Time Machine
disables third-party extensions
forces APFS consistency checks
rebuilds some security caches
Procedure:
Shut down
Hold power button → “Loading startup options”
Choose system disk
Hold Shift
Click Continue in Safe Mode
Let it fully boot, wait a minute, then reboot normally.
This alone often clears “fetching permissions” states.

Step 3 — Attempt manual mount after Safe Mode
After returning to normal mode:
Connect the ThunderBay
Mount using SoftRAID
When it mounts:
Immediately remove it from Time Machine (if not already done)
Backup if need be, for safety

Step 4 — Run Disk Utility First Aid (volume only)
On APFS, Disk Utility can:
checks APFS roles and security metadata
may clear flags SoftRAID does not touch
Run it on the APFS volume, not the physical disks.

Step 5 — If still not auto mounting: test on another Mac
Mounts elsewhere → macOS Sequoia issue on the Mac mini, try "Reinstall macOS" and see if that fixes this.

If it does not auto mount anywhere, you may need to start your Time Machine over (best is delete volume and create a new one, as there are no APFS repair tools for MacOS.

Thanks very much for your detailed reply. As it happens, I managed to solve everything except not the permissions issue a couple of days ago. This is what transpired.

The SoftRAID volume with the Time Machine backup is not the only SoftRAID volume on the ThunderBay. All the disks in the ThunderBay are HFS+ not APFS. I found this reply to someone else's problem: https://forums.softraid.com/softraid-8-functionality-issues/i-have-lost-availability-to-change-permissions-on-my-8-disk-28-tb-raid/#post-22755 . I paid Alsoft for a DiskWarrior license. DiskWarrior did not require the volume to be mounted in order to repair it. And DiskWarrior was able to reconstruct the directory and restore the volume.

This did not solve the permissions issue, but I will try your Step 2 to see whether the disk permissions can be sorted out.

@drolma-la 

As you noticed, Disk Warrior is fantastic for HFS.

Time Machine volumes must be APFS
(Except Pre Big Sur volumes, which used HFS. Since MacOS does not know how to convert an Apple RAID or SoftRAID volume to APFS, when upgrading from an older macOS to Big Sur and later, you must delete your SoftRAID volume and recreate it as APFS for Time Machine to work.)

Ah. Very interesting. My Time Machine backup was created when I was still running MacOS Big Sur. I haven't moved on to MacOS Tahoe yet, but Time Machine is working with the HPFS+-hosted backup under MacOS Sequoia. Thanks for drawing my attention to the new reality. And don't tell my significant other that I'm now considering adding yet another SSD to what is regarded as my over-paranoid backup arrangements.

@drolma-la 

You can never have enough backups.

We used to have a joke in the late 80's, "Real Men Don't Back Up", in a disk drive manual.